Help....I'm Stuck doing editing some basic code...browsing

I am NO Techie at all…I am trying to create a landing page from a template file and Opened the index.html file with NP++ BUT when it open and I see the code it can not be edited…what am I doing wrong…I’m sure that this very simple and I’m missing something easy…I have code for my own Paypal button and my own Youtube video to replace the others that are there.